In mammalian cells, sugar chains having a special structure reflecting each cell type, stage of development and differentiation, disease conditions, or the like are expressed. In the form of a glycoprotein or glycolipid, the sugar chains are present on a surface layer of a cell or in blood serum. Based on their properties, determination is made on their effective use as a biomarker like proteins and nucleic acids. In recent years, it is also found that the sugar chains play an important role in vivo, and thus studies on their function are also carried out. For example, “Siaα2,6Gal” or “Siaα2,3Gal” of a sialylated sugar chain is a sugar chain which is broadly present on a surface of lymphocyte involved with immunological response, and thus a pharmaceutical agent for detecting those sialylated sugar chains is employed as an indicator of immunodeficiency (Patent Literature 1).
However, compared to proteins and nucleic acids, the technique relating to structure identification or detection of sugar chains is not sufficiently established. Once an antibody capable of identifying and detecting a partial structure of the sugar chain, which specifies the structure of the sugar chain, is developed, it can be not only a huge advantage for fast and accurate diagnosis and development of a therapeutic agent for various malignant tumors and various symptoms, but also a simple sugar chain analysis based on ELISA or Western blot, both widely employed for protein analysis, can be achieved, and thus it is believed that further research and development regarding sugar chain function and industrial application can be promoted.
Among the oligosaccharide chains of an N-linked glycoprotein, the sugar chain structure represented by “6′-Sialyl-LacNAc (Siaα2,6Galβ1,4GlcNAc)” is also referred to as “CDw75”, and it is a structure shown in a glycoprotein or a glycolipid that is expressed in mammalian cells. This sugar chain structure is known to be an infection receptor of human influenza virus (Non Patent Literature 1), and it has also been identified as a cell surface layer marker of B-cell lymphoma (Non Patent Literature 2) or an epitope of a novel tumor marker (CDw75) having a relationship with the malignancy of gastric cancer or colorectal cancer (Non Patent Literature 3). Based on the relationship with those serious diseases, determination has been made on the effective use as a diagnosis indicator. In particular, it is highly expected to be used as a molecular target for determination and diagnosis of malignancy of gastric cancer or colorectal cancer, or treatment of malignant tumors such as B-cell lymphoma.
CDw75 is an antigen defined as a cell surface layer antigen that is commonly recognized by four kinds of a monoclonal antibody obtained by immunization of lymphocytes (Non Patent Literature 4), and it is expressed in mature B cells or in part of T cells in peripheral blood. As a partial epitope that is commonly recognized by those four kinds of the antibody, a sugar chain antigen represented by Siaα2,6Galβ1,4GlcNAc has been originally identified and found to be an essence of CDw75 (Non Patent Literature 3). However, it was also known that the antibody specificity is different among those four kinds of the antibody. It is believed that each antibody recognizes a different biomolecule with CDw75, including CDw75 and neighboring regions as an epitope. Among those four kinds of the antibody, most widely used antibody is LN-1 monoclonal antibody, which is also commercially available (Santacruz Biotechnology). However, due to insufficient affinity and specificity for CDw75, its use is limited to an immunohistochemical examination (Non Patent Literature 3).
When an oligosaccharide chain is used as an antigen, there is only insufficient development of an immune system which recognizes sugar chains in vivo of a mammalian. As such, it is generally difficult to produce an antibody which recognizes the oligosaccharide chain itself as an epitope. It is known that, although the sugar chains derived from microorganisms can be easily recognized, it is difficult to produce an antibody which recognizes a sugar chain antigen of a glycoprotein derived from mammalian, in particular, an oligosaccharide chain antigen included in N-linked sugar chains (Non Patent Literature 5 and Non Patent Literature 6). Even at the present moment, an effective antibody having the N-linked sugar oligosaccharide chain antigen itself of a glycoprotein as an epitope is hardly developed.
With regard to an antibody recognizing CDw75 as an epitope, there is a report of CDw75 monoclonal antibody which is obtained by a method of immune induction by using phosphatidyl ethanolamine as a carrier compound and using, as an immunogen, a compound obtained by conjugation of an oligosaccharide chain containing CDw75 in a reductive manner to the amino group of the carrier compound (Non Patent Literature 6). However, the obtained CDw75 monoclonal antibody has insufficient practical value as well as insufficient affinity and specificity.
As described above, although there is high expectancy for CDw75 sugar chain as a diagnostic agent or a molecular target for therapeutics, an antibody having practical value as well as high affinity and specificity, in which CDw75 itself is an epitope, does not exist up until now. Under the circumstances, development of an antibody with practical value enabling direct and accurate detection of CDw75 is waited for.